Chapter 1: The Copernican Revolution

The Motions of the Planets

The Birth of Modern Astronomy

The Laws of Planetary Motion

Newton’s Laws

Summary of Chapter 1

Units of Chapter 1

The Sun, Moon, and stars all have simple movements in the sky, consistent with an Earth-centered system.

Planets:• Move with respect to fixed stars• Change in brightness• Change speed• Have retrograde motion• Are difficult to describe in earth-centered system

1.1 The Motions of the Planets

A basic geocentric model, showing an epicycle (used to explain planetary motions)

Lots of epicycles were needed to accurately track planetary motions, especially retrograde motions. This is Ptolemy's model.

A heliocentric (Sun-centered) model of the solar system easily describes the observed motions of the planets, without excess complication.

1.2 The Birth of Modern Astronomy

Observations of Galileo:

• The Moon has mountains, valleys, and craters.

• The Sun has imperfections, and it rotates.

• Jupiter has moons.

• Venus has phases.

All these were in contradiction to the general belief that the heavens were constant and immutable.

The phases of Venus are impossible to explain in the Earth-centered model of the solar system.

1. Planetary orbits are ellipses, Sun at one focus.Kepler’s laws:

1.3 The Laws of Planetary Motion

2. Imaginary line connecting Sun and planet sweeps out equal areas in equal times.

Kepler’s laws:

3. Square of period of planet’s orbital motion is proportional to cube of semimajor axis.

Kepler’s laws:

The Dimensions of the solar system• The distance from Earth to the Sun is called an astronomical unit. Its actual length may be measured by bouncing a radar signal off Venus and measuring the transit time.

Newton’s laws of motion explain how objects interact with the world and with each other.

Newton’s first law:

An object at rest will remain at rest, and an object moving in a straight line at constant speed will not change its motion, unless an external force acts on it.

1.4 Newton’s Laws

Newton’s second law:

When a force is exerted on an object, its acceleration is inversely proportional to its mass:

a = F/m

Newton’s third law:

When object A exerts a force on object B, object B exerts an equal and opposite force on object A.

1.4 Newton’s Laws

Gravity

On Earth’s surface, the acceleration due to gravity is approximately constant, and directed toward the center of Earth.

Gravity

For two massive objects, the gravitational force is proportional to the product of their masses divided by the square of the distance between them.

Gravity

The gravitational pull of the Sun keeps the planets moving in their orbits.

Massive objects actually orbit around their common center of mass; if one object is much more massivethan the other, the center of mass is not far from the center of the more massive object. For objects more equal in mass, the center of mass is between the two.

Kepler’s laws are a consequence of Newton’s laws.

• First models of solar system were geocentric, but couldn't easily explain retrograde motion.

• Heliocentric model does.

• Galileo's observations supported heliocentric model.

• Kepler found three empirical laws of planetary motion from observations.

Summary of Chapter 1

• Laws of Newtonian mechanics explained Kepler’s observations.

• Gravitational force between two masses is proportional to the product of the masses, divided by the square of the distance between them.

Summary of Chapter 1, cont.